1. Field of the Invention
The invention relates to a method for producing a sterile, stable milk product with improved flavor capable of storage under ambient conditions, wherein sterility and stability of a milk fraction within the milk product is obtained using a combination of filtration and heat treatment of a raw milk portion.
2. Background
One of the greatest causes of economic loss of refrigerated foods is microorganisms, in particular psychrotrophs, and their enzymes. Psychrotrophic microorganisms flourish at temperatures which are typically used for food refrigeration, between about 0xc2x0 C. and about 15xc2x0 C. Heat treatments, such as pasteurization or sterilization, can kill most or all of these unwanted microorganisms and thus enhance the shelf-life of foods. However, enzymes produced by these microorganisms prior to heat treatment, as well as natural enzymes present in food even after pasteurization and/or sterilization, can provide a more intractable problem because many of these enzymes are markedly heat stable. Thus, these enzymes can cause subsequent spoilage even in a sterile food product, wherein all microorganisms are inactivated, dead or removed. This problem is of particular concern to the dairy industry for products stored at room, or ambient, temperature, where enzymatic activity is increased as compared to cold storage, which inhibits or significantly reduces enzymatic activity.
Traditionally, milk products including, for example, whole milk, 2% milk, skim milk, yogurt and cottage cheese have been stored, transported and sold in cold conditions, typically from about 4xc2x0 C. to about 8xc2x0 C., in order to prevent spoilage of the milk product due to microorganism growth. Despite the cold storage, certain microorganisms, in particular psychrotrophic bacteria, thrive, leading to milk product spoilage in a relatively short time, usually within several weeks.
Freshly drawn milk is likely to contain psychrotrophic bacteria contracted from milk handling equipment such as storage tanks and pipework, in addition to naturally occurring microorganisms which may include bacteria, mold and yeast. Psychrotropic bacteria have the ability to multiply relatively quickly at low storage temperatures, about 4xc2x0 C., which are employed by the dairy industry for raw milk. Such milk may be stored for up to 3 days at these temperatures before processing at the dairy. During this storage period, the psychrotrophs, and particularly Pseudomonas spp., can multiply to fairly high numbers, typically up to 10 million bacteria per milliliter of milk. This growth is accompanied by the production of extracellular enzymes. The extracellular enzymes, in particular proteases, attack milk proteins and produce breakdown products having a bitter taste. These enzyme activities are temperature dependent and increase with increasing temperatures.
Due to the speed at which microorganisms multiply, raw milk has a very short shelf life. The shelf life of milk is drastically reduced by the activity of the naturally occurring enzymes and enzymes produced by microorganisms present in the milk. This is true even in milk treated to reduce the number of microorganisms because some extracellular enzymes will be present in the milk before treatment intended to kill or inactivate such microorganisms can occur. Thus, it is essential that milk be conveyed quickly and while cold in order to minimize enzymatic activity.
The need for speed in conveying milk and milk-containing products to market is great because increasingly each step of milk processing to achieve consumer ready milk or milk products is done by a different entity. Thus, each milk product is transported and stored several times between drawing raw milk at the farm and placement of the milk product on the shelf at the local store. Each transport or storage is another period in which the milk or milk product may experience temperature fluctuation, and is another period during which microorganism growth and milk or milk product spoilage can occur.
An example of milk processing for cold storage milk is described in U.S. Pat. No. 5,935,632, corresponding to WO 96/36238. U.S. Pat. No. 5,935,632 describes a process of treating milk wherein the milk is separated into cream and skimmed milk portions. The skimmed milk is filtered with a filter of 0.05-2.0 xcexcm and then optionally treated at 50-72xc2x0 C. for up to 15 seconds. The milk is then pasteurized by heating to 70-85xc2x0 C. for 2-30 seconds or heating to 85-140xc2x0 C. for 1-10 seconds. Some of the filtered milk may be remixed with the cream portion and heated with the cream to 120xc2x0 C. for two seconds and then recombined with the heated and pasteurized milk. This process produces a milk product with an extended shelf life.
In order to reduce milk or milk product spoilage, it is desirable that milk be treated so that it may be handled under ambient conditions for long periods of time without spoilage. Treatment of milk to allow ambient storage and handling would greatly reduce the current costs of milk product production because there would be little or no need for refrigerated trucks or storage units during the production process, and the speed of transport and production would not be as critical. Further, the shelf life of milk products would be increased, thus greatly reducing wasted product.
Sterile milk, or commercially sterile milk, may be defined as a product which is free of microorganisms, such as bacteria and spores, which can grow under the prevailing storage conditions. Sterile milk, packaged under aseptic conditions in aseptic packages, enjoys the advantage of being able to be distributed and stored under ambient conditions for a lengthy period of time.
In order to produce such a commercially sterile milk, milk is commonly treated by UHT (Ultra High Temperature), i.e., treatment of the milk at an elevated temperature, normally from about 135xc2x0 C. to about 150xc2x0 C. for about 4 to about 15 seconds. As a result of this heat treatment, microorganisms are killed and enzymes are at least partially inactivated so that the resultant milk product has a shelf-life varying from about 3 to 12 months when stored under ambient conditions.
However, with UHT milk, enzymes, especially proteases, produced by microorganisms before heat treatment are of concern because, to a large extent, original enzyme activity and extracellular enzymatic activity can remain in the milk after UHT treatment. If sufficient enzymes are produced by psychrotrophs in the raw milk before heat treatment, the quality of UHT milk can deteriorate even after treatment, giving rise to degradation of the milk product during storage. Proteases are of especial concern because they can cause spoilage even when present at very low concentrations.
Another drawback inherent in milk treated by the UHT method is that the high temperature imparts a cooked flavor to the milk. The cooked flavor results at least in part from the heat-induced release of sulfhydryl groups in whey proteins, xcex2-lactoglobulin and euglobulin in the milk from the breakage of chemical bonds. See McGraw-Hill Encyclopedia of Science and Technology, 6th Ed., Vol. 11, (New York, 1987), p. 206. The degree of cooked flavor will depend on whether the UHT system used is direct or indirect. Direct systems produce a milk with less cooked flavor, but with a shorter shelf life when stored under ambient conditions than milk treated by indirect UHT at the same temperature and for the same amount of time.
Combinations of milk treatment by UHT and a lower temperature process of enzyme deactivation are described, for example, in GB 2209919A and U.S. Pat. No. 4,175,141. The enzyme deactivation can occur before or after UHT treatment, resulting in a sterile, stable milk product with an altered, cooked taste.
GB 2209919A describes a method of deactivating heat-stable enzymes in a nutrient such as milk or juice by subjecting the nutrient to UHT treatment at xe2x89xa7100xc2x0 C. for up to 2 minutes, cooling and holding the nutrient at a temperature of 45-95xc2x0 C. for 1-10 minutes, and then immediately cooling to less than 35xc2x0 C. within 5 minutes. The UHT treatment of milk preferably occurs at temperatures of at least 130xc2x0 C. for 1-10 seconds at a pressure above atmospheric pressure.
U.S. Patent No. 4,175,141 describes the UHT treatment of milk at 120xc2x0 C. or greater and low temperature treatment at 50-65xc2x0 C. for at least several minutes, preferably 5-60 minutes, to kill heat resistant proteases. The UHT and heat treatment can occur in any order.
UHT treatment is also known to be used in combination with filtration, as demonstrated in U.S. Pat. No. 6,117,470. This patent discloses filtration of skimmed milk with a filter having a pore size of 0.05-2.0 cm to form a permeate and a retentate. The retentate contains the major portions of fat and casein and is subjected to UHT treatment at 120xc2x0 C.-165xc2x0 C. before recombination with the permeate, homogenization and packaging for consumer consumption. This produces a skimmed milk with superior keeping qualities. However, improved flavor is still desirable.
Accordingly, there remains a need in the art to develop a process that provides a sterile milk with both improved flavor and good stability, thereby providing a long shelf life under ambient conditions.
A means of achieving a sterile, stable milk product with improved flavor capable of storage under ambient conditions, wherein sterility and stability of a milk fraction within the milk product is obtained using a combination of filtration and heat treatment of the raw milk portion, is described.
A method of obtaining a commercially sterile and stable milk product with improved flavor for consumer consumption with varying fat content is also described. The milk product is purer than milk products produced according to prior art methods because the invention provides a sterile and stable milk fraction for use in producing the milk product, wherein the milk fraction is free of heat resistant bacteria; free of, or has a reduced number of, thermoduric bacteria; and has a reduced number of heat sensitive bacteria.
One aspect of the invention is a process for forming a stable, sterile milk fraction wherein a raw milk portion is filtered, sterilized at from about 78xc2x0 C. to about 121xc2x0 C., stabilized at from about 50xc2x0 C. to about 121xc2x0 C., and enzymes are deactivated at from about 50xc2x0 C. to about 78xc2x0 C. The raw milk portion is not heated above 121xc2x0 C. during the process.
Another embodiment includes separating milk into a cream fraction and a raw milk portion; treating the cream fraction by UHT; filtering the raw milk portion to form a permeate; sterilizing the permeate at from about 78xc2x0 C. to about 121xc2x0 C.; stabilizing the sterile permeate at from about 50xc2x0 C. to about 121xc2x0 C.; deactivating enzymes in the sterile, stable permeate at from about 50xc2x0 C. to about 78xc2x0 C. to form a milk fraction; combining the sterile and stable cream fraction and milk fraction and homogenizing, wherein the raw milk portion is not heated above 121xc2x0 C. during processing.
Another aspect of the invention is the resultant sterile, stable milk fraction. The milk fraction can be produced by the processes described herein and is free of heat resistant bacteria and has a reduced number of both thermoduric and heat sensitive bacteria. Preferably, the milk fraction is free of heat resistant and thermoduric bacteria and has a reduced number of heat sensitive bacteria. The milk fraction is sterile and stable for at least about one month, preferably at least about two months, and more preferably at least about three months, under ambient conditions.
Another aspect of the invention is the milk product produced from the sterile, stable milk fraction. The milk product has a reduced number of heat resistant bacteria, thermoduric bacteria and heat sensitive bacteria as compared to prior art milk products. The milk product is stable for at least about one month, preferably at least about two months, and more preferably at least about three months, under ambient conditions.
Other aspects and features of the invention will be apparent to practitioners in the art upon review of the entire specification and claims.